【 Weak current College 】 PCB plating-nickel process and cause of the failure and exclude 】

1 the role and characteristics of

PCB (printed circuit board is English PrintedCircuieBoard abbreviation) with nickel-plated as precious metal and base metal substrate layer, single side printed circuit board for some, it is also often used as a topping. For some of the heavy load wear surface, such as switch-contact, touch tablet or plug gold, nickel as gold substrate coating, can greatly improve wear resistance. When used as a barrier, NI can effectively prevent between copper and other metals. Matt Nickel/Gold combination coatings are often used as anti-etching of metal coatings, and able to meet the thermal pressure welding and brazing, read-only nickel to as ammonia class Etchant corrosion Coatings without thermal pressure welding requires PCB plating brightener, usually uses Kwang nickel/gold plating. Nickel plating thickness is generally not less than 2.5 Micron, usually 4-5 micron.

PCB and low stress nickel deposition layer, usually with a modified form of watt nickel plating and reduce the stress of additives of Sulfamic Acid and nickel plating to plating system.

We often say PCB bright nickel plated nickel and Matt nickel (also called low stress nickel or half bright nickel), usually require detailed and uniform coating, porosity, stress low and malleable.

2. nickel sulphamate (ammonia-NI)

Nickel sulphamate is widely used as metal hole plating and printing plug contact chip coating of substrates. The deposition layer of low internal stress, high hardness, and has very good ductility. Will be a relief for joining plating, the coating will be slightly more stress. There are several different formulations sulphamate bath, typical nickel sulphamate bath formula below the table. Due to the coating of low stress, so get widely used, but nickel sulphamate poor stability, its cost is relatively high.

3. modification of watt nickel (sulfur nickel)

Modified formula, using nickel Watt nickel sulfate, nickel bromide with accession or nickel chloride. Since the causes of internal stress, so most of them use nickel bromide. It can produce a half bright, slightly little internal stress, ductility good coating; and such coating is then very easy activation of electroplating, costs relative to the bottom.

4, plating and the role of each component:

Main salt-amino-nickel and nickel sulphate for nickel plating on the main salt, nickel salts are mainly required for nickel plated nickel-metal ions and also plays a role in conducting salt. The concentration of nickel plating with supply manufacturers differ slightly different nickel content of salt allows. Nickel salts content is high, you can use the high current density cathodes, deposition speed, often used as high speed plating thickness nickel. But the concentration is too high will reduce the cathode polarization and dispersion of ability, and bath with large losses. Nickel deposition rate of salt content is low, but dispersed capacity low, access to Crystal careful bright coating.

Buffer-boric acid used as buffer for nickel plating of pH maintenance within a certain scope. Practice shows that when nickel plating of pH is too low and will make the cathode current efficiency decline; but when the pH is too high, due to the precipitation of H2, close to the cathode layer near the surface pH rapidly increased, leading to Ni (OH) 2, colloidal generation Ni (OH) 2 inclusion of the coating, plating embrittlement increase Ni (OH) 2 gel adsorption on the surface of the electrode, also cause hydrogen bubbles on the surface of the electrode, the detention of coating increases the porosity. Boric acid not only have a Ph buffer and he can raise the cathode polarization, thus improving performance, reducing bath in high current density of the phenomenon of "burning". The existence of boric acid is also conducive to improving the mechanical properties of the coating.

Anode Activator-apart from Kraft-nickel plating using insoluble anode, other types of nickel plating are soluble anode. While nickel anode in power process extremely easy passivation, in order to guarantee the normal dissolve, anode in the bath to a certain amount of anode Activator. Discovery through test, CI-CL-is the best nickel anode Activator. In the containing nickel chloride, nickel plating except as the primary nickel chloride salt and conducting salt, also played a role of anode Activator. Without nickel chloride or the lower anodised nickel plating, required under practical conditions add a certain amount of sodium chloride. Brominated nickel or nickel chloride also commonly used to make to stress agent used to maintain the coating of internal stress, and empowered and coating with half-bright appearance.

Additives: additives main ingredients are stress, stress removing agent, improved the plating of cathode polarization, lower coating of internal stress, as the stress concentration changes, you can make deposits internal stress by tensile stress change to compressive stress. Common additives are: naphthalene sulfonic acid, p-toluene sulfonamide, saccharin, and so on. And not to stress for nickel coating, plating isn't going to stress agent will be uniform and meticulous and have half a shiny coating. Usually to stress agent is one hour by AMPS to added (now common combinations of special additives including anti-pinhole agent, etc.).

Wetting agent — in the plating process, cathode on precipitation hydrogen is inevitable, hydrogen released from not only reduces the cathode current efficiency, but also because the hydrogen bubbles on the surface of the electrode stranded, will also enable the coating pinholes. Nickel layer of porosity is relatively high, in order to reduce or prevent pinholes, it should be added to the bath of wetting agent, such as sodium dodecyl sulfate, diethyl Hexyl sodium sulfate, sodium Octyl base, it is an anionic surface active substances that can be adsorbed on the surface, so that the cathode electrode and interface of the solution to reduce tension in the hydrogen electrode and wetting the contact angle is reduced, so that the bubble can easily leave the electrode surface, to prevent or minimize the generation of the pores.

5, bath maintenance

A) temperature — different nickel plating, the bath temperature is different. Temperature change on nickel plating process is more complex. At temperature higher nickel plating, nickel plating of low internal stress, malleable, temperature increase to 50 ° c with a coating of internal stress achieve stability. General operation temperature is maintained at 55-60 ° C. If the temperature is too highNickel will happen, the resulting brine solution of nickel hydroxide colloid colloidal make hydrogen bubble stranded, coating pinholes, while reducing cathode polarization. So the working temperature is very strict, should control the scope of the provisions, in the actual work is based on the vendor's optimal temperature value with room temperature controller to maintain its temperature stability.

B) Ph — practice showed that nickel electrolyte pH on coating performance and the performance impact of electrolyte. At PH ≤ 2 strong acidic electroplating solution, there is no metal nickel deposition, just released from light gas. General PCB plating-nickel electrolyte pH remained at 3-4. PH values higher nickel plating has a high dispersion of ability and higher efficiency of cathodic current. But when the PH is too high, because the cathode electroplating process constantly releasing light gas near the cathode surface coating of pH rises faster, when greater than 6, there will be light NIO colloid produce, resulting in the hydrogen bubbles stranded coating pinholes. Nickel hydroxide of inclusions in the coating, but also make deposits increased brittleness. PH low nickel plating, Anodic dissolution is better, you can improve the electrolyte salt content in nickel, allows the use of high current density, thereby strengthening the production. But the PH is too low and will allow for light coating of narrow temperature range. Join the carbonic acid nickel or nickel carbonate, Ph; join Sulfamic Acid or sulfuric acid, Ph decrease, in the course of every four hours to check the adjustment of the pH value at a time.

C) anode — now can see PCB conventional nickel plating are made of soluble anode, titanium baskets as anodes in nickel angle is quite common. Its advantages are its anode area may be large enough and does not change, the anode maintenance easier. Titanium baskets should be loaded polypropylene woven anode bags prevent anode slime away into the plating bath. And regular cleaning and inspection hole is clear. New anode bags before use and should be soaked in boiling water.

D) purification-when plating exist organic pollution, the activated carbon treatment should be. But this approach often part to stress agent removal (additive), must be added. The process is as follows;

(1) remove the anode, and cleaning water 5ml/l, heating (60-80 ° C) encouragement (gas mix) 2 hours.
(2) organic impurities, to join 3 — 5ml/30% hydrogen peroxide, lr, gas mixing 3 hours.
(3) 3 — 5g/l powder activity constantly stirring, continue gas mixing 2 hours, stirring to stand for 4 hours, standby tank filter powder used to filter both clear cylinder.
(4) cleaning and maintenance anode hang, a nickel coated corrugated iron as cathode, 0.5 — 0.1/spuare's current density for drag-and-cylinder 8-12 hours (when plating exist inorganic pollution impact quality, often used)
(5) changed the filter element (usually a group of cotton core set of carbon core series continuous filtration, according to the periodicity will change to a valid extension processing time, improving the stability of the solution), analysis of the various parameters, adjusting joined additive wetting agent to try plating.

E) analysis — plating solution should use technology to control the process of order elements, regular analysis of the plating solution components and hull trough test, based on the parameters guide production sector adjust various parameters of bath.

F) mixing-nickel plating and other electroplating process procedure, stir in order to speed up the transfer process to lower concentration, increase the permitted upper limit of current density. On the plating solution for mixing also has a very important role, is to reduce or prevent nickel layers produces a pinhole. Because, in the process of electroplating, cathode surface near the hydrogen ion plating, a large number of scarce precipitation, the pH increases from nickel hydroxide colloid, causing hydrogen bubble of detention arising from pinhole. Strengthening on the plating of mixing, eliminates the above phenomenon. Common compressed air, cathode mobile and forced circulation (in combination with carbon core and cotton core filter) mixing.

G) cathode current density – cathodic current density on cathodic current efficiency, deposition rate and coating quality. Test results show that when using the electrolyte PH lower nickel plated, low current density areas, cathodic current efficiency with current density increases; in high current density, cathodic current efficiency and current density-independent, but when using a higher PH nickel plating solution, cathodic current efficiency and current density.

As with other types of plating, nickel plating selected cathodic current density range should also be apparent electroplating components, temperature and mixing conditions, due to PCB Panel larger, high-current DC and low-current areas of current densities vary greatly, generally use 2A/dm2.

6, fault analysis and removal

A) Ma Hang: hemp pit is the result of organic pollution. Large Ma Hang usually indicates that there is oil pollution. Mixed performing, it cannot be expelled out of the bubble,which will form the Ma Hang. You can use a wetting agent to reduce its impact, we usually put the small pitting called pinhole, before bad, what quality with metal, boric acid content is too small, bath temperature too low will produce the pinhole, plating solution maintenance and process control is the key to preventing application of pinhole agent technology to additional stabilizers.

B) rough, Burr: rough explains solution dirty, full filtering can correct (PH is too high and easy formation hydroxide precipitation should be controlled). Current density is too high, the anode slime and additional water purification into impurity, serious will produce rough and burrs.

C) combined with low: If copper coating without fully to the oxide layer, the layer will be stripping phenomena, copper and nickel on the difference between the adhesion. If the current is interrupted, it will be interrupted, resulting in nickel coating itself, when the temperature is too low will also have to peel.

D) plating embrittlement, solderability difference: when the coating is bent or subject to a certain degree of wear, they often reveal plating embrittlement. This indicates the presence of organic compounds or heavy metals mixed quality pollution, additives, entrainment of organic matter and electroplating resist, is the main source of organic pollution, must be treated with activated carbon, addDHI is insufficient and the PH is too high can also affect plating embrittlement.

E) coating of dark and light color is not uniform: coating dark and light color uneven, with metallic contamination. Because usually first after nickel plated copper, so into the copper solution is a major source of pollution. It is important to keep the copper with hanging hook solution to a minimum. In order to remove the Groove metal pollution, in particular to copper solution should use corrugated steel cathode, in 2 ~ 5 Ann/square feet of current density, per gallon solution empty plating 5A for one hour. Before dealing with the bad side, low coating, current density is too small, the main salt concentration is too low, electroplating power circuit bad will affect the coating color and luster.

F) coating burn: the burn caused coating possible causes: insufficient Borate, metal salt concentration is low, the operating temperature is too low, the current density is too high, the PH is too high or mixing.

G) deposition rate low: pH low or low current density will cause deposition rate low.

H) coating blistering or peel: pretreatment adverse, Middle power off time is too long, organic contamination, current density is too large, the temperature is too low, the PH is too high or too low, the effect of severe impurities will produce blistering or skin.

I) anode passivation: anode Activator deficiency, anode area is too small for the current density is too high.